HR: 1340h
AN: T33A-1328    [Abstracts]
TI: Magnetic anisotropy at the lithosphere-asthenosphere boundary: preliminary results from the Ronda peridotite, Spain
AU: Martin-Hernandez, F
EM: fatima@geo.uu.nl
AF: Utrecht University, Paleomagnetic Laboratory Fort Hoofddijk, Utrecht, CD 3584 Netherlands
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Garrido, C J
EM: carlosg@ugr.es
AF: Universidad de Granada, Departamento de Mineralogia y Petrologia, Facultad de Ciencias, Campus Fuentenueva s/n, Granada, 18002 Spain
AU: Belley, F
EM: francebelley@yahoo.fr
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AB: The Ronda massif (S. Spain) is a large (300 km2) orogenic peridotite massif located in the Betic Cordillera, the westernmost part of the peri-Mediterranean Alpine orogen. One of the unique features of this massif is the coarsening front which marks the transition between two petrostructural and geochemical domains: [D1] an intensely foliated porphyroclastic spinel peridotite domain with subordinate garnet pyroxenite layers and [D2] a coarse-granular spinel peridotite domain with minor spinel pyroxenite layers. The coarsening front, which extends for 20 km along strike, is marked by a strong grain size increase and annealed microstructures. Based on previous olivine LPO data acquired using EBSD, deformation in [D1] occurred at a significantly lower temperature <$1000\deg$C than in [D2] ($1250\deg$C). Tectonic fabrics between [D1] and [D2] are parallel. The macroscopic foliation dims gradually from [D1] to [D2] but compositional layering and spinel trails (lineation) persist in [D2]. Totally recrystallized melting residues of former garnet pyroxenite dykes (now spinel websterites) display folded structures inherited from the spinel tectonites. Geochemical and textural data show that the coarse-grain peridotite domain [D2] does not represent a "true" asthenospheric mantle but rather derived from the lithospheric spinel tectonite domain [D1], through re-heating, partial melting and pervasive interaction with basaltic melt at quasi-asthenospheric conditions (ca. $1250\deg$C at 1.5 GPa). The increase in fabric intensity, from the tectonites to the coarsening front, that is deduced from olivine LPO data is currently unexplained. Magnetic measurements were performed on 80 specimens (20 mm cubes) from 10 stations. The low field magnetic susceptibilities and degree of anisotropy indicate the dominance of serpentinization-related secondary MD magnetite. The low field AMS fabrics are oblique to the mineral foliation and lineation. Measurements conducted in high magnetic field, using a vibrating sample magnetometer, reflect the contribution of paramagnetic minerals (mostly olivine and orthopyroxene). Torsion magnetometry measurements conducted at fields up to 1.8 T show the lack of high coercivity phases such as hematite. Preliminary results show that the low field AMS and the high field AMS principal axes (K1 and K3) are statistically distinct. These preliminary results allow to constrain the origin of the increase in fabric observed at the lithosphere-asthenosphere boundary.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting